The present invention relates to an air separation process and apparatus for producing high purity nitrogen. More particularly, the present invention relates to such a process and apparatus in which compressed, purified, and cooled air is separated in an air separation unit incorporating high and low pressure columns. Even more particularly, the present invention relates to such a process and apparatus in which the high purity nitrogen is produced in the high pressure column and reflux is supplied to the low pressure column through indirect heat exchange between the high purity nitrogen and tower overhead produced in the low pressure column.
Many industrial processes require high purity nitrogen. For instance, the semiconductor industry uses high purity nitrogen as a carrier gas, a drydown gas, an inerting gas, and etc. High purity nitrogen is produced through the cryogenic distillation or rectification of the air in one or more columns. When oxygen production is also required, an air separation trait is utilized that has high and low pressure columns operatively associated with one another in a heat transfer relationship by a condenser-reboiler. In such apparatus, air after having been compressed, purified and cooled to near dewpoint temperatures is introduced into the bottom of the high pressure column. In either of the columns, contacting elements such as trays, plates, packing, either structured or random, are used to bring an ascending vapor phase into intimate contact with a descending liquid phase. As a result of such contact, the ascending vapor phase has an ever increasing nitrogen concentration as it ascends within the column and the descending liquid phase has an ever increasing oxygen concentration as it descends within the column. In the high pressure column, an oxygen-enriched column bottom is produced and a high purity nitrogen vapor tower overhead is produced. The high purity nitrogen vapor tower overhead is condensed against boiling liquid oxygen produced within the low pressure column to supply reflux for both the high and low pressure columns.
In order to utilize the high purity nitrogen vapor tower overhead to supply reflux to the low pressure column the low pressure column must also produce a high purity nitrogen vapor tower overhead and as such, the low pressure column must incorporate a sufficient height of packing or a sufficient number of trays or plates to produce the required nitrogen refinement. Thus, part of the initial capitalization of a double column high purity nitrogen plant is expanded in the construction of a low pressure column designed to produce high purity nitrogen.
As will he discussed, the present invention provides a process and apparatus for producing a high purity nitrogen product through the separation of air in a double column air separation unit that does not require the production of high purity nitrogen in the low pressure column. This allows a low pressure column of the present invention to be constructed with less packing or fewer trays than similar columns of the prior art. The advantage of this can be realized in reduced plant construction costs.